Classic Hifi Care

Nice job, and thank you for the tip on balance control repair. I have a record player that does the same thing, loud left and right but soft in the middle. I may just try your suggestion. Of course I have no idea what resistance to use, mine is a PP tube unit and is a Magnavox.

Bill

The choice of transistor was the result of Google AI providing some silicon types that were commonly used for germanium replacements, plus some other videos and articles that I found. I ordered an assortment from Amazon that had the 2N3906 along with 9 other types, (250 transistors for only $7!!!). I figured I'd be able to use one of those others if it didn't work out. I expected some trial and error, but it just worked. One guy on ARF said it would probably work in that circuit. But I know there are some circuits you would have to tweak the bias resistors, like the P-P output transistors, for instance.

I never found any reason for the oscillation, which is frustrating, but the caps fixed it without hurting anything. I do think the balance pot was just bad design, but maybe the oscillation was the result of aging components in a design that had marginal stability in the first place, not that I could have done any better back then!

Regarding the balance resistor, I just figured something in the range of 10 percent of the value of one side of the pot might work, and it did, but I had already tried adding 10K resistors on each end of the pot, which really made the oscillation much worse! So these solutions were not without many days of research plus trial and error all around the whole amp. I'm glad it's over (I hope).

Keeping my fingers crossed that it is over for you, at least with this project. And, thank you for the tip on adding the resistor to the balance pot. I need to work on my Maggie with the same issue, it now has woofer flutter in both channels no matter what input I use.

Bill

Replaced the cartridge in the changer with the Pfanstiehl P-226D in Gary's kit, managed to get all the tiny fiddley pins soldered, had to widen the slots in the provided plastic mount, plus the little screws were longer than in the pictures of the instructions provided, but after all that it sounds good and tracks properly. So the original EV cart was indeed defective, I think the compliance was just shot, even though electrically it was OK. Set tracking at about 4 grams, a little higher than I wanted, but seems that what it needs. Changer detects sizes and drops records correctly, after adjusting the spindle pusher. The service manual has been very helpful!.

Unfortunately, the amp has started that oscillation issue again, not as bad as before, so I need to investigate that again. I had been using my test speakers for a while instead of the OEM ones; the oscillation reappeared when I reconnected them. Plus, there's some low frying sounds from time to time, probably coming from the two germaniums in the preamp that have not been replaced, so I'll try that.

Meanwhile, I've also started cleaning up the cabinet. Will discuss that later....

The unit is missing it's Magnavox emblem, I think it may have looked like this (screenshot photo from a different model). The holes in my unit look like this, the two holes on the left are 1" apart, the third one is 1 1/8" from the middle one. I saw some joker on eBay offering two for $250.00, and they're plastic!

Anybody have any suggestions on where to find one?

It's been a while since I posted my progress on this guy, but I've pretty much finished up so here's the latest.

The oscillation issue kept raising its ugly head:-I realized that there was still a low-level 8-10 Hz background oscillation that was present almost all the time, could only see it on the scope. But, depending on the music content, sometimes it would crank up and cause nasty distortion in the audio. I tried everything I could think of: experimenting with different cap and resistor values in the feedback loop, the front end circuits and the speaker crossovers. Nothing would make a positive impact, only made it worse! The only thing that completely stops the oscillation is bypassing the 2uF crossover caps to the tweeters. Even put back in the OEM crossover caps, same problem. So I have settled on just bypassing the 2uF caps, leaving in the series 4 ohm resistors to the tweeters. I actually can't hear any difference in the sound with the cap in or out. The amp only puts out a watt or two, so I don't think the bass is going to burn up the tweeters.

Now to the cabinet--boy this thing is heavy! All plywood/Tolex. I stripped off the roached (water damaged) brown Tolex on the end pieces and gently pried out the gold plastic trim that separated the two colors. Scrubbed the heck out of the rest of the tan Tolex, but nothing would remove the grime that was embedded in the crevasses of the pattern. So, maybe it's heresy, but I ended up spray painting the tan Tolex with some Ace satin enamel in ivory color, after patching some of the damage and little dents. For the end caps, I picked up some brown Deco Vinyl Fabric from Hobby Lobby (on sale, less than $3 worth!), glued with contact cement. The original grille cloth was also totally unsalvageable. But I had a lot of "Sahara" colored fabric from an old Coolaroo "shade sail" and it works great as a grille cloth, actually pretty acoustically transparent! The latch that secured the fold down door had one broken leaf spring inside, but after some internal mods, I was able to make it work with a compression spring from a ball point pen. Cleaned the rust off the hardware, looks a lot better! The inside of the changer compartment is white Tolex, I cleaned it up and was able to touch up the ripped area with some white glue, plus white-out mixed with the ivory enamel.

Also added a Bluetooth receiver, will discuss that next.

Hey, looks good!

Chris Campbell

I've bought several types of Bluetooth adapters from Amazon over the years, but the one I installed on this project works really well. It has a "3D" button which sounds like it boosts bass and treble, and that makes it sound really good on this unit. After some experimenting, I ran the Bluetooth audio output (low impedance) directly into the tone tap on the loudness control instead of switching it into the phono input. That approach loads the front end such that the audio from the cartridge is suppressed as long as the Bluetooth is powered up, but when power is removed from the Bluetooth adapter, its output becomes a high impedance, so it has no effect on the audio from the cartridge. I added a small switch so you can disconnect the power to the 5volt USB Bluetooth supply when playing a record. When the switch points to the changer, it turns off Bluetooth. The switch (red) is behind the control escutcheon, located just under the Magnavox emblem. You can see it in the front view photo in the previous post.

Finally, I added a fuse and power cord to the opening on the rear of the cabinet. Originally, it had a removable power cord, but that had been ripped out long before I got the unit. The inside brackets above the changer were for storage of the original cord.

Sound wise, I think it suffers greatly from the side firing speaker arrangement. Sounds pretty dull when listening to a record from a position in the front since all the treble is shot out sideways. I'm also wondering if the replacement cartridge has different enough characteristics compared to the OEM cartridge, perhaps the front end was "tuned" for the original EV cartridge's impedance characteristics. The bass control has to be fully clockwise to sound even halfway decent. It basically functions by adjusting the input impedance seen by the cartridge, maximum bass is the highest resistance, and even disconnecting it to give a maximum input impedance sounds the same as in- circuit at max. Definitely a low cost method for creating a bass control!

Listening to Bluetooth is much more satisfying, especially with the option of using the "3D" button. Of course, the unit's tone controls are non-functional in that case.

The components of this unit are capable of much better fidelity if installed in a better designed cabinet as seen in other portables of this era, or in a console. I'll probably end up selling it, or donating it to our local charity thrift store.